The use of spinal fixation instrumentation to align and/or fix a desired relationship between adjacent vertebral bodies is well established. Such instrumentation typically includes a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to pedicle screws which have been inserted into the patient's vertebrae or to spinal hooks which can be placed into a vertebral arch for coupling to the vertebral bodies. Once installed, the spinal fixation instrumentation holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
One example of a rod based spinal fixation system is provided in U.S. Pat. No. 5,005,562, issued Apr. 9, 1991 to Cotrel (which is hereby incorporated by reference). This system includes pedicle screws and spinal hook vertebral coupling elements (both screws and hooks) having integral U-shaped bodies that extend outward from the vertebrae to which they are attached. A spinal fixation rod is shaped as desired and fitted into the “U” of U-shaped bodies of adjacent vertebrae. The inner surfaces of the U-shaped body are threaded to accept a set screw, and rod is fixed to the vertebral coupling elements by threading a set screw into each of the U-shaped bodies to lock in the rod.
U.S. Pat. No. 5,545,165, issued Aug. 13, 1996 to Biedermann et al. (and incorporated herein by reference), illustrates an improvement in closure systems for fixing a rod to vertebral coupling elements over those provided by Cotrel. The Biedermann et al. system also uses pedicle screws and spinal hooks having U-shaped bodies that extend outward from the vertebrae to which they are attached. The U-shaped bodies of the Biedermann et al. system are threaded on both the inside and the outside. The rod is therefore locked in by both an inner set screw and an outer lock nut. In the illustrated embodiments, the inner set screw is adapted to be driven on its threads using a hex-shaped driver element, and the outer locking nut is provided with hex-shaped flat outer surfaces suitable for engagement with a wrench or similar driving tool.
U.S. Pat. No. 5,443,467, issued Aug. 22, 1995 to Biedermann et al. (and incorporated herein by reference) illustrates the use of an inner set screw and an outer lock nut to lock a rod into a U-shaped body in a polyaxial screw system. The use of these two elements to lock a rod can result in a “four point contact” between the two locking elements and the rod—that is, each of the inner set screw and outer lock nut can contact the rod in two places, resulting in four contact points that lock the rod down. In this system, a pedicle screw having a spherical head is captured within a separate U-shaped receiver body. The angle of the screw with respect to the body can be changed until a head-locking element is tightened to lock the angle of the screw head within the receiver body. According to Biedermann et al., this combination of an inner set screw and an outer locking nut provides an advantage in that the force acting on the rod can be independently adjusted by either the inner set screw or the outer locking nut—a particularly useful advantage where the rod being fastened is curved and an exact fastening might only be possible by independent adjustment of the two closure elements. In addition, when tightened, the inner set screw and the outer locking nut tend to lock each other in their tightened positions.
While the closure systems of the Biedermann et al. patents have been quite successful, the illustrated embodiments necessarily involve the tightening of two separate elements using two separate tools. It would be beneficial to provide a single locking nut assembly that could lock a rod down with a four point contact, but require only a single tool and a single locking step by the surgeon.